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I had a call on a Carrier furnace 58SE175-3. How do I tell if the flame sensor is bad? I can't recall how to set my multimeter to test it. The problem is the over temp is shutting burners down before blower comes on. Any ideas?

I'm not positive if that particular model is sensing flame by rectification (single wire going to little bar in the flame), but from what I've read so far, it seems it probably does.

To test, you set your multimeter to microamps DC. Your meter has to be in series with the flame sensor. You will usually read anywhere between 2-8 uADC on your meter when the flame is enveloping the rod. Most controls need to see between .5-1 uADC for flame sensing.

In other words, the control board is looking for the amperage, or flow of current, to sense a flame. This is completely different than a thermocouple which does actually generate millivoltage to hold a magnet in place to keep the gas valve open.

By the way, as the flame sensor gets "dirty", the microamperage will decrease. A brand new furnace will usually, in my experience, generate 5-7 uADC. When you see it dropping dangerously close to the 1 uADC minimum, you might want to try clean the rod.

I'm not sure what type of furnace this is either but to start, if it is a counterflow, make sure your filters are clean. Not enough return air and your limit will trip. Circuit board could be malfunctioning and not starting fan in time. If there is no circuit board whatever controls the fan may not be working properly. Thermostat heat anticipator fried or set wrong, limit switch malfunctioning. Check temp at limit when it shuts down. Good luck

I think they had a cam stat style of limit and a G60 control on the gas valve or mounted above the burners.

When I suspected a flame signal problem I would just clean the flame sensor and fire it up.

If the limit is triping you might try to stick a probe in place of the limit and see how hot it's getting. Could be a bad camstat. I rember replacing more that I thought I should have the last couple of years I work for that Carrier dealer. Seemed like poor quality.

Those furnaces had a coating in the HX. Look close for any rust lines. They could be cracks. Look hard at the dimples and water test if possible.